1. Field of the Invention
The present invention relates to a row of continuous slide fastener coupling elements in the form of a method of manufacturing a filamentary coil or zigzag made of thermoplastic synthetic resin.
2. Prior Art
As is well known in the art, a row of continuous slide fastener coupling elements is made by bending a monofilament of thermoplastic synthetic resin successively into a plurality of coils or turns each including a coupling head, a pair of spaced legs extending from the coupling head, and a connector located remotely from the coupling head and extending between one of the legs and a leg of an adjacent coil or turn. Since the monofilament is resilient, it is customary to heat-set the coiled or turned monofilament so as to relax internal stresses, so that the bent configuration may be maintained against further dimensional change. Such heat-setting has been performed bodily on the monofilament either by heating a die for forming the monofilament turns or coils or by moving the bent monofilament through a heated atmosphere.
It is generally known that a thermoplastic material can be well heat-set for increased dimensional stability by heating the material at a temperature close to its melting point. However, a coiled or otherwise bent monofilament cannot be heated to such a temperature because the monofilament, when thus heated, is liable to become deformed while it is withdrawn from the forming die. For this reason, the monofilament when it is shaped into coupling elements, is temporarily heat-set at a temperature well below the melting point so as to allow it to become sufficiently cooled with the coupling elements are issued out of the die. For such temporary heat-setting, the monofilament should be subjected to heat generally for 30 seconds during which molecules in the monofilament are stably rearranged due to Brownian movement. With this procedure, a zone for heat-setting must be lengthened for speeding up the element-forming operation, or the element-forming operation must be slowed down for making the element-forming device smaller in size. Further, such heat-setting procedure requires a great amount of thermal energy to be consumed for heating the formed coupling elements wholly and for forcibly cooling them down.
A pair of rows of coupling elements thus heat-set at a relatively low temperature are interlocked and attached to a pair of slide fastener stringer tapes, which then are dyed. The rows of interengaged coupling elements, or a chain of coupling elements, are finally heat-set during such a dyeing process. For example, the stringer tapes made of cotton or Nylon 66 are normally dyed in a coloring agent heated to a temperature of about 95.degree. C. that is considered to be equivalent to a temperature of 145.degree. C. in atmosphere, which is substantially equal to or below a temperature (about 140.degree. C.-150.degree. C.) at which the coupling element rows have been heat-set. However, with the stringer tapes being made of polyester, they should be dyed at a temperature of about from 130.degree. C. to 140.degree. C., the atmospheric equivalent of which exceeds the temperature of heat-setting of the coupling elements. This usage has led to a drawback in that the coupling elements tend to become deformed, resulting in disturbances in element-to-element distance or pitch, twists in the rows of coupling elements, or variations in the height or thickness of the coupling elements. Accordingly, to minimize the above difficulties, care should be exercised as to selection of the temperature at which the coupling elements are to be heat-set, the material of the monofilament, or the properties thereof, such as the rate of water absorption or shrinkage.
With the foregoing disadvantages in view, it has been attempted to apply ultrasonic energy wholly to the monofilament being shaped within a relatively short period of time, such heat-setting being performed at a temperature below and close to the melting point of the monofilament. With such ultrasonic heat-setting, however, a great amount of energy must be consumed, and an expensive ultrasonic oscillator that necessitates a high degree of precision is required, making the formed coupling elements more costly.